The invention relates generally to the field of radio-navigation receivers and more particularly to a method for mitigating constructive interference in a received radio-navigation signal by modeling the interference, and then subsequently removing the interference from the signal.
An example of a radio-navigation satellite system (RNSS) is the United States Global Positioning System (GPS). The GPS was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted at L1 and L2 respectively. These signals include timing patterns relative to the satellites onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
GPS""s designers assumed that all transmitters would be aboard satellites at a large and relatively constant distance from all user receivers, consequently generating signal levels at the receivers that would be weak and relatively constant. This assumption drove a number of trade-offs in system and satellite transmitter design and continues to influence receiver development even today.
Despite this assumption ground-based transmitters (known as PLs, pseudo-satellites, or simply pseudolites) have been used to complement the GPS satellites from the very beginning. In the foreseeable future, PLs may be incorporated in Unmanned Aerial Vehicles (UAVs). A PL transmits a signal with code-phase, carrier-phase, and data components with the same timing as the satellite signals and the same format. A GPS receiver acquires such a PL signal and derives code-phase pseudo-ranges or carrier-phase measurements to be used in a navigation algorithm in substantially the same manner as for a GPS satellite. The major differences are that a PL typically does not contain a high-accuracy atomic clock and that the PL position must be described in geographical terms rather than in orbital elements.
Precision navigation and landing systems require reliable and highly accurate position, velocity and time information (these aggregately denoted herein as PVT information) not achievable by standalone GPS. Precision-guided weapons require reliable PVT information to achieve acceptable Circularly Error Probable (CEP) targeting errors. To meet these requirements additional radio-navigation transmitters are needed. These transmitters can be additional satellites as specified in the Wide Area Augmentation System (WAAS) or PLs based on the ground as specified in the Local Area Augmentation System (LAAS), or on board ships, or even UAVs loitering in the air above an area of interest. WAAS and LAAS can transmit either correction data (i.e., differential data) or provide additional ranging information. When these transmitters use the GPS spectrum, as is the case for UAVs, PLs, and satellites providing ranging information, additional interference is added. This constructive interference is seen as noise to the receiver, which can degrade and in some cases prevent a receiver from acquiring and tracking the satellites.
Moreover, introduction of PLs violates one of the key assumptions of the designers of GPS. Thus, the distance between a user receiver and a PL can be large or quite small, so PL signal levels at a receiver can vary significantly. Relatively strong PL signals have the potential to overwhelm satellite signals and jam a receiver, whereas weak PL signals may be too feeble to allow receiver tracking. This is the basis for a wireless communication difficulty known in the art as the xe2x80x9cnear-farxe2x80x9d problem.
Equally problematic is the sharing and encroachment of the GPS radio frequency spectrum from other users. Mobile Satellite Systems (MSS) downlinks, wind profiler radar, space based radar, ultra-wideband systems, GPS expansion and the European Radio Navigation Satellite System known as Galileo, have or have filed for frequencies in and around the GPS spectrum. These additional systems are potential sources and targets of interference from and to existing RNSS systems.
Another type of interference is self interference, which is the result of signals from a radio-navigation transmitter interfering with the reception of radio-navigation signals at the receiver. This type of interference often occurs when a RNSS receiver and transmitter are located physically near (or identical to) each other. Self interference is an extreme case of the xe2x80x9cnear-farxe2x80x9d problem.
Another RNSS interference concern is spoofing and meaconing. Spoofing is a technique for causing an active radio-navigation receiver to lock onto legitimate-appearing false signals and then be slowly drawn off the desired path causing significant PVT errors. In addition, spoofers can effectively jam large geographical areas. Meaconing is a technique for the reception, delay, and rebroadcast of radio navigation signals that can confuse a navigation system or user.
In general, spoofing is more difficult to achieve than generic jamming, and is often targeted to an individual user. Spoofing, however, can achieve the same effect (widespread disruption) as jamming. This is because a spoofer can inject misleading data within a localized area and its pseudo-random number (PRN) signal will act as a highly effective jammer over large distances. A spoofer can defeat nearly all anti-jamming equipment.
In conclusion, many types of radio navigation interference exist within the RNSS RF spectrum, and it is desirable to have a method and apparatus to identify and remove such wireless interference that compromises the usefulness of legitimate radio navigation signals. In particular, it would be advantageous to reduce or mitigate the near-far problem in radio navigation.
The following abbreviations are used herein.
ADC: Analog to Digital Converter
AFRL: Air Force Research Lab
C/A codexe2x80x94Coarse/Acquisition or Clear/Acquisition Code
CDMAxe2x80x94Code Division Multiple Access
CEPxe2x80x94Circular Error Probable
DARPAxe2x80x94Defense Advanced Research Projects Agency
DGPSxe2x80x94Differential GPS
DLLxe2x80x94Delay Locked Loop
DOPxe2x80x94Dilution of Precision
E codexe2x80x94European code
DSPxe2x80x94Digital Signal Processing
FLLxe2x80x94Frequency Locked Loop
GNSSxe2x80x94Global Navigation Satellite System (ICAO definition)
GPSxe2x80x94Global Positioning System
IFxe2x80x94Intermediate Frequency
IMUxe2x80x94Inertial Measurement Unit
INSxe2x80x94Inertial Navigation System
LAASxe2x80x94Local Area Augmentation System
MASxe2x80x94Multiple Access System
MFDxe2x80x94Matched Filter Detector (Technique used in most GPS receivers)
MSDxe2x80x94Matched Subspace Detector (Technique used in DFC next-generation receiver)
MSSxe2x80x94Mobile Satellite System
NFxe2x80x94Near Far
NFRxe2x80x94Near Far Resistant
P(Y) codexe2x80x94Precision (Encrypted) code
PLxe2x80x94Pseudolite, pseudo-satellite
PLLxe2x80x94Phase Locked Loop
PRNxe2x80x94Pseudo Random Noise code, e.g., C/A Gold codes and the P(Y) codes.
PVTxe2x80x94Position, Velocity, and Time
RAIMxe2x80x94Receiver Autonomous Integrity Monitoring
RFxe2x80x94Radio Frequency
RNSSxe2x80x94Radio Navigation Satellite System
ROCxe2x80x94Receiver Operating Characteristic
SAxe2x80x94Selective Availability
SNRxe2x80x94Signal to Noise Ratio
SVxe2x80x94Space Vehicle (e.g., an RNSS satellite)
VCOxe2x80x94Voltage Controlled Oscillator
UAVxe2x80x94Unmanned Aerial Vehicles
UMPxe2x80x94Uniformly Most Powerful
USAFxe2x80x94United States Air Force
WAASxe2x80x94Wide Area Augmentation System
It is advantageous to define several terms before describing the invention. It should be appreciated that the following definitions are used throughout this application. Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.
GPS Codes: Each GPS satellite or PL at least transmits two different codes such codes typically include: a coarse/acquisition (C/A) code and a precision (encrypted) (P(Y)) code. Each C/A-code is a unique sequence of 1023 bits, called chips, which is repeated each millisecond. The duration of each C/A-code chip is about 1 micro-second. The chip width or wavelength is about 300 meters. The rate of the C/A-code chips, called chipping rate, is 1.023 MHz (or magachips/sec or Mcps). A P-code is a unique segment of an extremely long (≈1014 chips) PRN sequence. The chipping rate is 10.23 Mcps, i.c., ten times that for a C/A-code, and the chip width is about 30 meters. The smaller wavelength results in greater precision in the range measurements than that for the C/A-codes.
Wireless Signal Model:
Let a wireless navigation signal y be modeled as follows:
y=Hxcex8+Sxcfx86+nxe2x80x83xe2x80x83(1)
where
H is a representation of the desired or target signal (i.e., a vector) for which
interference is to be diminished;
xcex8 is the amplitude on the target signal H;
xcfx86 is a vector of amplitudes applied on the matrix S of interference signals;
n is the noise and S
S=[s1s2 . . . sN]xe2x80x83xe2x80x83(2)
s1 is the interference signal i.
Near-Far Interference: The commingling of two or more different wireless signals from one or more wireless sources in such a manner that when the commingled signal is received, one of the signals is sufficiently strong (and likely from a source nearer to the receiver) so that it overwhelms a weaker commingled signal (likely from a source, e.g., farther from the receiver). In particular, the stronger signal may xe2x80x9cleakxe2x80x9d such that there is sufficient signal cross correlation to compromise the accurate detection of the weaker signal.
Near-Far (NF) interference can occur both in military and civilian environments and be from friendly or hostile sources. Friendly sources include PseudoLites (PLs) placed at, for example, airports to enhance navigation. Unfortunately the strong PLs signal can actually interfere with the receiver""s ability to acquire and track the satellite signals thereby unintentionally denying the ability to navigate.
Hostile interference is found in the military arena where a hostile force deploys ground or air based PLs with the intent of confusing GPS receivers within an area. Any type of military hardware that uses a GPS receiver is susceptible. This includes: aircraft, vehicles, command and control, and even GPS guided munitions.
Structured interference: Is any wireless (e.g., radio) interference source whose signals can be predictively modeled.
Self-Interference: Wireless signal interference that occurs when a receiver is collocated (i.e., located within proximity sufficient to induce interference) with a transmitter. Self interference is the result of signals transmitted from a radio-navigation transmitter interfering with a radio-navigation signal received on the same antenna as used for transmitting. This type of interference often occurs when a receiver doubles as a transmitter. Self interference is an extreme case of xe2x80x9cNear-Farxe2x80x9d interference.
Spoofing, Meaconing and Jamming Interference: Spoofing is used to cause an active radio-navigation receiver to lock onto legitimate-appearing false signals and then be slowly drawn off the desired path causing significant PVT errors. In addition, spoofers effectively jam large areas. Meaconing is the reception, delay, and rebroadcast of radio navigation signals to confuse a navigation system or user.
Higher Order DLL, FLL, PLL: Generally, the order of a phase locked loop (PLL) is 1 higher than the order of the loop filter. If the loop filter is omitted, i.e., if the output of the phase detector directly controls a voltage controlled oscillator (VCO), a first-order PLL is obtained. The term xe2x80x9corderxe2x80x9d is defined herein as the exponent on the largest term in the filter polynomial. As one of ordinary skill in the art will understand, higher-order loop filters offer better noise cancellation, so loop filters of order 2 and more are used in critical applications.
Massively parallel acquisition scheme: This is a system which can at least substantially continuously acquire the signals of interest. It""s ability to divide the Doppler, phase and code offset search space is only limited by the number of correlators and speed of the processors. In theory such a system could provide the interference modeling parameters to the present invention.
Navigation Data: GPS transmitters transmit a navigation data message which includes: a telemetry word, hand-over word, clock corrections, SV health/accuracy, ephemeris parameters, almanac, ionospheric model and coordinated universal time data.
Nominal Satellites: Satellites that are operating normally, e.g., within their design specifications.
Processing Channel: A processing channel of a GPS receiver provides the circuitry necessary to process the signal from a GPS transmitter (e.g., a satellite, or pseudolite). In general such a channel is where the acquisition and tracking functions take place.
Steady State: A computational state of an embodiment of a GPS receiver according to the present invention, wherein at least most and typically all the initial interfering signals (collectively denoted as xe2x80x9cSxe2x80x9d) have been identified (i.e., xe2x80x9clabeledxe2x80x9d), and at least most and typically all the signals Q (interferers and non-interferers alike) have achieved xe2x80x9cgood lockxe2x80x9d by the GPS receiver. Note that (xe2x80x9cgood lockxe2x80x9d denotes the estimates of Doppler, phase and code offsets that are varying within an acceptable range (e.g., one set of experiments indicated phase must be within 12 degrees of truth, Doppler must be within 28 Hz of truth, and code offset must be within {fraction (1/50)} of a chip).
The present invention is a method and system for reducing radio navigation interference so that a radio navigation receiver can more effectively detect and utilize legitimate wireless navigation signals as well as mitigate, cancel and/or remove interfering wireless signals.
The present invention is applicable to any radio-navigation system in which the interference to be removed can be predictively modeled (e.g., interference whose structure is known and can be simulated). In particular, the present invention is applicable with radio-navigation systems such as: United States GPS, and the proposed European Galileo navigation satellite system, to name a few.
It is an object of the invention to provide a technique that is independent of how a radio-navigation signal is transmitted. For example, the present invention can be applied to substantially any wireless frequency, e.g., radio frequencies: L1, L2, L5, E1, E2, M1, M2. In addition, the present invention can be applied to any Pseudo Random Noise (PRN) code, e.g., Coarse Acquisition (C/A) code, Precise P(Y) code, the new Military M-codes, and even the to-be-defined E-codes.
It is yet another object of the invention to have radio-navigation receivers equipped with an embodiment of the invention to thereby be resistant to the Near-Far interference problems and special cases thereof such as self-interference, jamming and spoofing.
It is yet another object of the present invention to be usable with and transparent to existing navigation augmentation and landing systems, e.g. WAAS, LAAS, and Inertial Navigation Systems (INS). Furthermore, should these augmentation and landing systems provide ranging information, the present invention can be an integral part of their architecture.
It is yet a further object to provide embodiments of the present invention that are fully compatible with all current Receiver Autonomous Integrity Monitoring (RAIM) techniques. RAIM provides timely warnings to GPS receiver users when the integrity of their PVT solution has been compromised. The various RAIM techniques are all based on some kind of self-consistency check among the available measurements. To be effective RAIM requires redundancy of information, i.e., 5 satellites to detect an anomaly and 6 satellites to identify and remove its faulty data from the navigation solution. Accordingly, the present invention adds an additional integrity monitoring technique for detecting and preventing spoofmg and meaconing. Moreover, a wireless navigation receiver equipped with an embodiment of the invention only needs four satellites to detect a spoofer or meaconer, and only five satellites to remove their faulty data from a navigation solution.
It is also an object of the invention to provide a technology to make radio-navigation receivers more robust to interference. Embodiments of the invention can operate within a radio-navigation receiver as a signal processing technique. Additionally, embodiments of the present invention can be effectively used on analog or digital signals, and on RF or IF ranges. Thus, if predetermined and/or predictable structured interference is present in a wireless navigation signal, such interference can be removed and/or mitigated, and the resultant signal is passed to acquisition and tracking routines, as one skilled in the art will understand.
It is a further aspect of the present invention that it can be embodied substantially in software, firmware or other programmable techniques within a GPS receiver having appropriate hardware to enable the signal processing performed by the present invention. Moreover, it is also an aspect of the present invention that substantially all processing performed by the invention is embedded within the logic of one or more special purpose hardware components (e.g., chips, logic circuits, etc.) substantially without the need for programming such hardware components. Of course hybrid embodiments that are between a substantially programmed embodiment and a substantially hardware embodiment are also within the scope of the present invention.